Cobalt transforms into fission product .sup.60 Co when exposed to radiation. Accordingly, it is desirable, from the viewpoint of safety, to use a super alloy free from cobalt or containing as little cobalt as possible for use in such portions of a high temperature gas cooled reactor where materials are exposed to radiation and members connected thereto.
However, most of the conventional super alloys having an extremely long creep rupture life at high temperatures of about 1,000.degree. C. contain cobalt. On the other hand, the conventional super alloys free from cobalt usually have an insufficient creep rupture strength. Further, with conventional super alloys, inclusive or exclusive of cobalt, there are very few which have plastic formability sufficient to produce such products as bars, sheets, plates and pipes from the materials cast from such alloys.
Accordingly, a super alloy free from cobalt which has an excellent creep rupture strength at high temperatures of about 1,000.degree. C. and a plastic formability over a wide range of temperatures and which is particularly suitable for use as a structural material for a high temperature gas cooled reactor has been desired, but no super alloys having such properties as mentioned above have been developed as yet.